I deleted all my mods so I can just have the standard game, when I went to play it (after installing it again) a load of NPCs are bald, I've tried closing the game down and reinstalling it but they're still appearing bald, and I really od not want to go through every household and add hair to them, its really irratating.

Well, you got rid of your custom content and evidently you had a lot of custom hair enabled for random, because that's the only way NPCs would have it. So, you're gonna have to give them all hair, or else delete them and let the game spawn new Sims to take their places. In the future, get Sims 4 Studio and run the batch fix to disable all CAS custom content for random, and you won't have bald (or naked) NPCs again.

How do I delete them? Do I just evict them out of their houses??

Evict them, however many you need to, then go into Manage Worlds and find the households (whether they're played or unplayed) and you can delete them from there. You need to make sure you evict them first.

May i ask why you must evict them first? I never evict them first. I just go to manage world, manage households and delete whatever families i want to delete, from there.

I know evicting them makes them leave the furniture in the house. But it seems like there was another reason to evict them first and now (of course) I can't remember what it is.

well if ever it comes back to you, let us know please.
In the meantime, I'll try to remember to evict them first, just..because!

I looked around on Google and I just can't remember what it is. Now this is bugging the crap out of me. Seems like it's a bug of some kind.

Had the same problem, and the way I found to solve it was through cheats.

First, CTRL + SHIFT + C, type "testingcheats true", then SHIFT + NPC you want to give hair to. It'll have an option along the lines of "Edit in CAS".. Click on it, and it'll open CAS for that character's household. You can edit only said character or everyone in the household, if there are any and/or you want to edit someone else. Give them any hair from the game and you're good to go.

Works for any character you're not playing with, even if they're not an NPC. I had to do that with the postwoman just yesterday, but also did it with my sim's girlfriend from a different household.

If you ever want to use CC again, just use Sims4Studio to make sure any hair you have in your Mods won't be allowed for random.